Hum bucking circuit



Dec. 23, 1952 R. H. DREISBACH ET AL 2,623,127

HUM BUCKING CIRCUIT Filed Feb. 1, 1950 5 Sheets-Sheet l INVENTORS Dec. 23, 1952 R. H. DREISBACH ET AL 2,523,127

HUM BUCKING CIRCUI T 3 Sheets-Sheet 2 Filed Feb. 1, 1950 LuJL \ INVENTORS:

awn/M06935 icb Dec. 23, 1952 H. DREISBACH ET AL 2,523,127

HUM BUCKING CIRCUIT Filed Feb. 1, 1950 3 Sheets-Sheet 5 M 1.1 E )))M(( PC l M T;

IOOO N current) Patented Dec. 23, 1952 UNITED STATES 'ATENT OFFICE HUM HUCKING CIRCUIT Delaware Application February 1, 1950, Serial No. 141,752

11 Claims. 1

This invention relates to transducing apparatus "and more particularly to hum bucking circuits for use therein where hum bucking is ad-- vantageous or necessary to suppress or eliminate hum resulting from unwanted alternating currents as occurs, for example, in dynamic loudspeakers, magnetic phonograph pickups, shorted turn testers and the like.

The invention will be first described by way of example in connection with 'a dynamic loudspeaker, and secondly, by 'way of example in connection with a shorted turn tester.

The direct current fieldexcitation in the field coil of a dynamic loudspeaker'is usually obtained from a power source which not only provides the desired direct current, but also superimposes thereon a certain alternating current (ripple This is due to the fact that common filters used in such sources 'to obtain the direct current are not completely able to filter the alternating current in such a way that only a direct current without a ripple current is obtained. This is especially true in the case of radio receivers and like apparatus in which the field coil (used to produce the necessary magnetic field) of the dynamic loudspeaker is used as the filter choke of said source.

Accordingly, an undesired voltage is induced into the moving coil of the dynamic loudspeaker, which results in an undesired alternating current when the moving coil is terminated by an impedance of a second source supplying the desired alternating current to the moving coil in order to reproduce the desired sound. The undesired alternating current causes objectionable hum,

Many attempts have been made in the past to eliminate this hum, but they have only been partially efiective and far from satisfactory. These methods have used individually and in combination hum bucking coils, copper rings and steel rings. A hum bucking coil is a special coil comiected in series opposition with the moving coil. It is usually placed around the centerpole of the field structure, either at the rear end of the field coil, or under it, or at the front end of it.

Also, copper rings have been used at the front end of the field coil next to the moving'coil, but if these copper rings are large enough to serve as an effective shorted turn to substantially reduce the undesired alternating current flux through the moving coil (periodical alteration of the magnetic field caused by the before mentioned undesired alternating current in the field coil) they have the disadvantage of also re- '2 closing the inductance of the field con to such an extent that the field coil can no longer serve as an effective filter choke if the field coil is used as a choke in radio receivers and like anparatus.

Steel rings also have a shorted turn effect, but increase leakage in the field, especially when used at the front end of said coil; there is accordingly a considerable reduction of the useful field through the moving coil resulting in less efficiency of the loudspeaker.

Also, copper or steel rings in combination with bucking coils have been used, the desired combination being only found by a lengthy and costly trial and error method.

None of these arrangements has proved to be really successful, the degree of success depending upon the design of the particular field structure involved and many field structures responding very poorly to the above treatment.

If the turns of a bucking coil were adjusted so that the same hum voltage appeared across it as appears across a moving coil associated with it, it is to be expected that when the two coils are connected in series opposition, no hum voltage would appear across the combination and hence, no undesired hum current would flow if the combination was connected to an impedance of the source supplying the desired alternating current, thereby forming a closed circuit.

However, in practice, it has not been found possible to buck the two mentioned hum voltages in the bucking coil and the moving coil to a satisfactory minimum and it has been generally recognized that this is due to a phase difference between the two voltages and a wave shape difference.

It is an object of the present invention to overcome the above difficulties and disadvantages and provide means which will prevent the flow of hum current through the moving coil.

To this end by this invention it has been determined that the difference in wave shape between the two voltages is of minor importance the two wave shapes do not differ too much, and that the problem is principally one of terresting the phase of' the voltage induced in the bucking coil by said flux.

Whereas others have unsuccessfully attempted to correct this condition by changing the flux through the bucking coil, as hereinbe'fo're explained, according to the present invention the problem is approached by shifting the phase of the hum voltage after it is induced in the bucking coil and then bucking this phase corrected hum voltage against the hum voltage of the moving coil, thereby eliminating flow of hum current through the moving coil. The bucking coil can then be located in the field structure where it is most convenient. A necessity is that the bucking coil circuit connected in series with the moving coil must be of low impedance with respect to the moving coil, otherwise too much of the signal power of the source supplying the desired alternating current will be dissipated, resulting in loss and reducing the efiiciency of the loudspeaker.

In order to shift the phase of the hum voltage of the bucking coil there is provided an impedance across the bucking coil thereby forming a closed circuit in which will flow a hum current, the closed circuit including at least one reactance element. This current develops a hum voltage across one of the elements of this circuit, which is equal in phase and magnitudeto the hum voltage induced in the moving coil with which it is connected in phase opposition. Thus there is no hum voltage appearing across the combination of the moving coil and the bucking coil circuit and therefore no hum current will flow through the moving coil when this combination is connected to the impedance of the source supplying the useful moving coil current.

It is possible to obtain the desired result by using as an impedance a capacitor across the bucking coil as this circuit will shift the phase, but the impedance of the usual bucking coil in a dynamic loudspeaker is so low that the capacitor has to have a value of several hundred microfarads. This is impractical because of the size of such a capacitor.

It has been found that an inductance in series with the bucking coil feeding a load resistor will give the desired phase shift to the voltage appearing across the resistor resulting in no fiow of hum current through the moving coil. It is obvious that the additional inductance required may be built into the bucking coil itself by using additional turns (thereby increasing the inductance) so that it is necessary to add only a resistor. This is a preferred form of the invention.

The advantages of this invention will be more clearly understood from the accompanying drawings embodying the invention in which:

Figure 1 is a longitudinal sectional view of a dynamic loudspeaker embodying the invention;

Figure 2 shows the usual hum bucking open circuit;

Figure 3 is a vector diagram corresponding to the circuit of Figure 2; i

Figure 4 shows an open'circuit embodying the invention;

Figure 5 is a vector diagram corresponding to the circuit of Figure 4;

t ili'igure 6 shows the circuit of Figure 4 in de- Figure I is a vector diagram corresponding to the circuit of Figure 6;

Figures 8a, 8b, 8c, 8d, 8e, 8 8g, 8h, 81', 87' and 810 showing some of the possible hum bucking coil circuits embodying the invention.

Figure 9 shows a typical shorted turn tester not embodying the invention.

Figure 10 shows a shorted turn tester embodying the invention.

Figure 1 shows a dynamic loudspeaker comprising a frame I, a field coil body structure 2, of U-shape in longitudinal section and preferably of circular cross section. Centrally disposed within the field coil body structure 2, and securely fixed thereto, is a cylindrical bar of magnetizable material 3, this bar extending forwardly through the central opening 8, in the field coil body structure. Within the field coil body 2 is located the field coil 4, its turns lying in a plane perpendicular to the longitudinal axis of the bar 3, said field coil being insulated from the field coil body structure 2, by an insulating layer 5. Between the field coil 4 and the rear end portion of the field coil body structure is provided a space I8 for a hum bucking coil BC insulated from the field coil body structure 2 and field coil 4 by means of an insulating layer 6. An annular air space I is provided between the iron bar 3 and the front end portion of the field coil body structure 2, to provide space for a movin coil MC, on a body 9, which body is preferably of cylindrical shape. The turns of the moving coil MC are located in a plane parallel to the plane of the turns of the field coil 4. Attached to the body 9 is a cone I 0, which serves to reproduce the sound in'the well-known manner. A shield ll attached to the cone III is provided to protect the air space I from dust and dirt as is usual. Elastic means H are provided for centering the moving coil body 9, one end thereof being attached to the frame I by means of a support 13, and the opposite end to the body 9. Wires M are provided for connecting the moving coil MC to the source supplying the desired alternating current to the moving coil MC in order to obtain sound reproduction in the usual manner. A resistor l5 together with the bucking coil BC and conductors [6 form the hum bucking circuit according to the invention. Suitable means I! are provided for attaching the resistor l5 to the field struc ture body 2.

In Figure 2 is shown the usual connection of the bucking coil BC and moving coil MC, that is to say, the two coils are connected in series opposition. In the bucking coil BC is a voltage Es induced by the alternating hum current in the field coil and induced in the moving coil MC is a voltage Ev. The vector diagram of Figure 3 indicates that the voltages EB and Ev do not have the proper desired phase difference, Ev being the moving coil voltage lagging the bucking coil voltage Ee by an angle p, and -Ev being a vector representation of the relation when Ev and EB are considered in phase opposition.

The invention in a preferred embodiment uses the basic circuit shown in Figure 4 in order to derive a hum voltage from the hum bucking coil which is opposite in phase to that of the movin coil, Figure 5 being the corresponding vector diagram of Figure 4. In Figure 4, BC indicates the bucking coil. To obtain a voltage from EB which will lag EB by and be in phase with Ev add an inductance L and a resistor R as follows: The additional inductance is connected in series with the bucking coil BC, and the resistor R is connected in parallel with the combination of the inductance and the bucking coil. Using the terminals of the resistor R. the moving coil MC is connected inseries with the closed circuit consisting of the coil BC. the inductance Landthe resistor R. In the coil BC is the voltage EB induced by the alternating hum current in the field coil, and induced in the moving coil MO is the voltage Ev. For the purpose of illustration in Figures 4 and 5, the inductance and resistance of the bucking coil BC are lumped in with the inductance L and resistance R which are considered as the lumped values L and R in the closed circuit,- Figure 5 illustrates how a. voltage IR may be obtained from the resistance component which will be in phase with the voltage: Ev. In addition to being of the same phase it is also necessary that the two voltages be of equal magnitude; however, the total IR component is not available for connection thereto, as a part of the resistance is inherent within the bucking coil. Therefore, in order to increase the value of the available IR drop and yet maintain the required ratio of L to R, it is necessary to increase the bucking coil voltage EB so that it is still greater than the voltage Ev. This is accomplished by increasing the number of turns in the buckin coil BC.

According to the present invention it ispossible in a dynamic loudspeaker having a buckingcoil to calculate with a fair degree of accuracy the value of the required resistor and the number of turns to be added to the bucking coil in order to obtain the desired result, and even provide the required additional inductance within the bucking coil itself, thus eliminating the need of an inductance separate from that of the bucking coil.

To do this it; is first required that the following data be known (the symbols refer to Figures 6 and 7) 1. Numberof turns of the bucking coil of a dynamic loudspeaker not embodying the present invention (ma).

2. Resistance value of said bucking coil (RE)- 3. Inductance value of said bucking coil (LB).

4.. Hum voltage induced across said bucking coil (EB) 5. Hum voltage induced across the moving coil of the loudspeaker (Ev).

6. Net voltage when said bucking coil and said moving coil voltages are connected in series opposition.

All this data can be easily obtained by the usual Well-known measuring methods.

From physics is known that the Voltage and resistance vary directly as the number of turns in the bucking coil. The inductance of the bucking coil varies as the square of the number of turns. Although this is not exactly true, as other factors are also involved, nevertheless, the resulting error in the following computation is so small that it can be neglected.

Figure 6, which is the circuit of Figure 4 in detail, shows the bucking coil BC divided into an inductance portion LB and a resistance portion RB. When additional turns are added to the bucking coil BC, this introduces in the circuit additional inductance L and additional resistance R". The induced voltage EB is raised by introducing inductance L to {'NB/nB}.EB;nB being the number of bucking coil turns originally, that is to say, before introduction of L, whilst NB ex presses the number of bucking coil turns after introduction of L. The total value of the in.- ductance in the hum bucking circuit is therefore {NB/nB} .LB, whilst the value of the original resistance and additional resistance in the circuit is {NB/its}.Rs. The total value of the resistance in the circuit isv {NB/HB}-.RBIR0 (Re being the value of the load resistor).

The current flowing in the closed circuit is represented by I.

The moving coil MC consists likewise of an inductance portion L and a. resistor portion R. The induced voltage being Ev.

Figure '7 shows the vector diagram corresponding to the circuit of Figure 6. The voltage on 6 resistor R0 being: IRQ. whilst the voltage. on the resistor Re and 13. together is {Ns/nsl-lial.

In vector Re.+{ B.I, I e must equal Ev, {Ns/nB} .cLB.I being the total voltage on. the inductancev LB and L together, and in which m=21r9f (1'' being the frequency of the alternating current causing; hum).

Vector (Ne/m1) .EE is the total voltage induced in. the bucking coil aftercorrection of the bucking. coil. EB; represents, the magnitude of this vectorbefore correcting the voltage of the bucking coil by increasing its number of turns. Calculation to find an expression for Ne/ns and for Re, if; Ev, Es, w, RB, o, LB are known data-z Consider Eigure 7.:

solving for Re then Ns/ne is determined from (4) Before correction the net voltage of the moving coil and bucking coil was, .0049 volt. After correction this dropped to .0005 volt which represents an improvement of about 20 decibels as the hum current is reduced by about this same factor.

Figures 8a to 870 show a plurality of possible hum bucking circuits embodying the invention. In applications other than dynamic loudspeakers it may be preferable to use one of these circuit arrangements, as may be dictated by such considerations as the sign and magnitude of the phase difference, the impedance of the device, space limitations, and ease of adjustment.

Figures 8a to 8f inclusive show bucking coil phase correcting circuits for a moving coil voltage Ev lagging bucking coil voltage EB, whereas the circuits shown in Figures 89 to 8k inclusive show circuit for a moving coil voltage Ev leading bucking coil voltage EB- Figure 8a shows a capacitor [8 parallel with the bucking coil BC. Figure 8b shows a resistor IS in series with a capacitor parallel with the bucking coil BC. Figure 8c shows an inductance 21 in series with the resistor 22 parallel to the bucking coil. Figure 801 shows an inductance 23 in series with a capacitor 24 parallel with the bucking coil BC. Figure 8e shows a resistor 25 and a capacitor 26 in parallel, and in series therewith is an inductance 21, the whole combination being parallel with the bucking coil. Figure 8f shows an inductance 28 in series with a variable resistor (potentiometer) 29 parallel with the bucking coil BC. Figure 8g shows an inductance 30 parallel with the bucking coil BC. Figure 8h shows resistor 3| in series with an inductance 32, the combination being parallel to the bucking coil BC. Figure 82' shows a capacitor 33 in series with a resistor 34, the combination being parallel to the bucking coil BC. Figure 87' shows a capacitor 35 in series with an inductance 3B, the combination I being parallel to the bucking coil BC. Figure 8k shows an inductance 38 and a resistor 39 in parallel, and in series therewith a capacitor 37, the whole combination being parallel with the bucking coil BC. In the circuits 8e, 8e, 82' and 87c it is possible to use instead of non-variable resistors, variable resistors (potentiom-eters).

Figure 9 illustrates a shorted turn tester of Well known construction in which PCI and PCZ are pickup coils, usually being identical, and connected in opposition.

The coils PCI and P02 are located on a magnetic core M. The magnetic core M has on one side an extension E to provide space for a test coil TC to be tested on shorted turns. Located substantially in the middle between the pickup coils PCI and PCZ is a field coil FC. Through the field coil FC flows an alternative current from a source supplying alternative current preferably having a frequency of 1000 cycles. Voltages induced in the coils PCI and PCZ by the flux of the field coil are led to the inputside of an amplifier A. Due to connection in series opposition of the coils PCI and P02 zero sign-a1 output is obtained at the outputside of the amplifier. Therefore, an indicator I connection to the outputside of the amplifier indicates no bias. The degree of balance between coils PCI and PCZ determines the sensitivity. However, in common practice a perfect balance cannot be obtained because of a phase difference between the voltages induced in the coils PCI and P02 which is due to the un symmetrical magnetic circuit.

The use of a phase correcting circuit in accordance with the present invention is shown in Fig. 10 connected to coil RC2. This circuit will correct the phase difference and will. permit the use of a magnetic circuit best adapted to the physical application of the apparatus.

The phase correcting circuit includes an additional inductance element L and a resistor element R.

The shown circuits are not all of the possible circuit arrangements. It is therefore to be understood that the invention is not limited to the circuits shown and described, as it is obvious that various other arrangements may be provided which fall within the spirit and scope of the invention, and within the ambit of the appended claims.

We claim:

1. In an electro-mechanical transducing apparatus having a coil; means for suppressing hum in said coil comprising a closed circuit including a bucking coil and a reactance element, said circuit comprising a phase shifting impedance connected to said coil so that the voltage in said hum bucking circuit is in opposition to the voltage in said coil.

2. In an electro-mechanical transducing apparatus having a coil; means for suppressing hum in said coil comprising a closed circuit including a bucking coil and reactance elements, said circuit comprising a phase shifting impedance connected to said coil so that the voltage in said hum bucking circuit is in opposition to the voltage in said coil.

3. Electro-mechanical transducing apparatus according to claim 2, wherein one of said reactance elements is a variable resistor.

' 4. In an electro-mechanical transducing apparatus having a coil and a hum bucking coil; means comprising a hum bucking impedance circuit for shifting the phase of the hum voltage induced in said hum bucking coil to a degree sufficient to suppress the hum voltage in said coil when said coil is connected in series to said circum.

5. In electro-mechanical transducing apparatus having a coil and a hum bucking coil; means for eliminating hum after hum voltage has been induced in said coil, comprising a closed hum bucking circuit, reactance elements in said circuit having an impedance for shifting the phase of the hum voltage induced in said hum bucking coil to a degree sufficient to suppress the hum voltage in said coil when said coil is connected in series with said circuit.

6. In electro-mechanical transducing apparatus having a voice coil having a hum voltage and a hum bucking coil for generating a hum bucking voltage equal to the hum voltage in the voice coil; means for eliminating hum after hum voltage has been induced in said coil, comprising a hum bucking circuit, fixed reactance elements in said circuit, said reactance elements being an additional inductance element in series with said bucking coil and a fixed load resistor element parallel with said bucking coil and said additional inductance element for shifting the phase of the hum voltage induced in said hum bucking coil, whereby to suppress the hum voltage in said coil when said coil is connected in series to said circuit.

'7. An electro-mechanical transducing apparatus as claimed in claim 6, wherein said-reactance elements have a predetermined value.

8. In an electro-mechanical transducing apparatus having a voice coil having a hum voltage, and a hum bucking coil for generating a hum bucking voltage equal to the hum voltage in the voice coil; means for eliminating hum after hum voltage has been induced in said coil, comprising a circuit, fixed reactance elements in said circuit, saidreactance elementscomprising an inductance element in series with said bucking coil and forming with said bucking coil, a hum bucking coil having additional turns, and a fixed resistor element parallel with said hum bucking coil having additional turns for shifting the phase of the hum voltage induced in said hum bucking coil, whereby to suppress the hum voltage in said coil when said coil is connected in series to said circuit.

9. In an electro-magnetic transducing apparatus having a first coil, a field coil and a second coil; a closed circuit, including said second coil having an impedance for shifting the phase of the voltage induced in said second coil by said field coil to a degree suflicient to equalize the voltages induced in said first mentioned and said second coil.

10. In an electro-magnetic transducing apparatus having a first coil, a field coil and a second coil; a closed circuit and reactance elements in said circuit, said reactance elements constituting an additional inductance element in series with said second coil and a load resistor element parallel with said second coil and said inductance phase of the voltage induced in said second coil 10 by said field coil to a degree sufiicient to equalize the voltages induced in said first mentioned coil and said second coil.

11. An electro-magnetic transducing apparatus as described in claim 10, wherein said load resistor element is variable.

ROBERT H. DREISBACH. CLIFFORD L. HARDVVICK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,105,924 Pridham et al. Aug. 4, 1914 1,535,538 Maxfield et a1. Apr. 28, 1925 1,830,401 Meissner Nov. 3, 1931 1,830,402 Meissner Nov. 3, 1931 2,190,318 Kellogg Feb. 13, 1940 FOREIGN PATENTS Number Country Date 272,622 Great Britain June 20, 1927 456,027 Great Britain Nov. 2, 1936 

